1. Field of the Invention
The present invention relates to manufacturing methods of a thin film transistor, a liquid crystal display device, and a semiconductor device.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as integrated circuits (ICs) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices, and the like. In an active matrix liquid crystal display device, a voltage is applied between a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation is recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
The application range of the active matrix liquid crystal display devices is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix liquid crystal display devices have high reliability and that a production method of the active matrix liquid crystal display devices offers high yield and reduction in production cost. As a method for increasing yield and reducing production cost, simplification of the process is given.
In active matrix liquid crystal display devices, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photolithography processes or simplification of a photolithography process are important for simplification of the whole process. For example, if one photolithography process is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like. In addition, steps before and after the aforementioned steps such as film formation, etching, resist removal, cleaning, and drying are needed. The number of steps is significantly increased only by adding one photolithography process in the manufacturing process. Therefore, many techniques for reducing the number of photolithography processes or simplification of a photolithography process in a manufacturing process have been developed.
Thin film transistors are broadly classified into top gate thin film transistors in each of which a channel formation region is provided below a gate electrode and bottom gate thin film transistors in each of which a channel formation region is provided above a gate electrode. In general, at least five photomasks are needed for manufacturing such a thin film transistor.
Many conventional techniques for simplification of a photolithography process use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There is a possibility that the yield is decreased due to various problems caused by usage of such a complicated technique. Moreover, in many cases, electric characteristics of thin film transistors are sacrificed due to the priority of the simplification.
As typical means for simplification of a photolithography process in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 is disclosed, for example.